1. Field of the Invention
The present invention relates to a keysheet module used in small electronic devices such as mobile phones.
2. Description of Related Art
Small electronic devices such as mobile phones have conventionally incorporated a keysheet module having parts, such as dome-shaped contact switches and LEDs, mounted on a flexible printed circuit board (FPC board). The keysheet module is arranged on the underside of a keypad formed integral with a plurality of key tops (see prior art of Japanese Patent Disclosure No. 2003-151391).
FIG. 3 illustrates an example construction of a conventional keysheet module. In the figure, a keysheet module 1 comprises a printed circuit board 20 formed of glass epoxy resin, a plurality of dome-shaped contact switches 3 provided on an upper surface of the printed circuit board 20, fine wiring patterns 4 formed on the printed circuit board 20, LEDs 5 arranged appropriately between the wiring patterns 4 to illuminate the keypad, electric elements 6 such as resistors and capacitors, a white spacer 80 as a reflection plate fixedly bonded to an upper surface of the printed circuit board 20 to reflect upward the light emitted from the LEDs 5, a transparent switch fixing sheet 90 fixedly bonded to the upper surface of the printed circuit board including the spacer 80 to cover the upper surface of the contact switches 3 for their secure holding, and a flexible connector 10 having power and signal lines to supply electricity to the keysheet module 1 and to send and receive signals. The spacer 80 of the keysheet module 1 is formed with part mounting holes 8c at positions corresponding to the LEDs 5 and electric elements 6 so that the LEDs 5 and electric elements 6 protrude from the part mounting holes 8c. 
In such a conventional keysheet module, its components such as contact switches, LEDs, and electric elements including resistor and capacitors need to be arranged in a limited space to meet a demand for reduced weight and size, however, as a result, it makes the wiring patterns very complex. For example, to connect separate wiring patterns formed on a double-sided printed circuit board without crossing (short-circuiting) other wiring patterns in between, it is necessary to form wiring patterns on both of top and bottom surfaces of the printed circuit board 20 and connect them via through-holes, as shown in FIG. 4.
The connection between top and bottom wiring patterns on the double-sided printed circuit board will be explained in detail by referring to FIG. 4. On the printed circuit board 20 there are formed wiring patterns 4e, 4f, 4g a predetermined distance apart from each other. To connect the wiring patterns 4e, 4g without crossing (short-circuiting) the wiring pattern 4f lying therebetween, conventional practice involves forming a pair of through-holes 22, 22 in the printed circuit board 20 and another wiring pattern 21 on the back of the printed circuit board 20 and connecting the wiring patterns 4e, 4g via the through-holes 22, 22 and the wiring pattern 21.
When such a double-sided printed circuit board is used, the need to form these through-holes 22, 22 and back-surface wiring pattern 21 increases cost compared with a one-sided printed circuit board.
Thus, most of the conventional keysheet modules are known to be constructed by a one-sided printed circuit board rather than a double-sided printed circuit board. FIG. 5 and FIG. 6 show separate wiring patterns on a one-sided printed circuit board connected together three-dimensionally by a chip jumper crossing over a third pattern (see Japanese Patent Disclosure No. 2003-45701).
In FIG. 5 and FIG. 6, three separate wiring patterns 4h, 4i, 4j are formed on the printed circuit board 20. On this printed circuit board 20, a coverlay 17 as an insulating layer is bonded to the upper surface of the circuits and the wiring pattern 4i. Between the wiring patterns 4h, 4j a chip jumper 23 with an inner resistance of 0 Ω is provided. The chip jumper 23 has a body portion 23b enclosed on its circumference with an insulating material and connecting terminals 23a, 23a at both ends of the body portion 23b, with the connecting terminals 23a, 23a secured to the wiring patterns 4h, 4j respectively with solder 24 to three-dimensionally connect the wiring patterns 4h, 4j together over the wiring pattern 4i. 
In the above method that uses the chip jumper 23 on the one-sided printed circuit board, however, since the volume of the chip jumper 23 is large, the chip jumper 23 when placed on the printed circuit board 20 is unavoidably arranged at positions that block the light emitted from the LEDs 5, as shown in FIG. 3, as in the case of the electric elements 6, thereby significantly degrading a keypad illumination efficiency.
Further, for the chip jumpers 23 to be arranged on the printed circuit board 20, the spacer 80 needs to be formed with part mounting holes for the chip jumpers 23. This in turn reduces a reflection area for the LEDs 5, also degrading the keypad illumination efficiency.